The photovoltaic device refers to a device that can convert photon energy into electric signals through a certain physical phenomenon (photovoltaic conversion). Solar cells, which are a type of photovoltaic device, can efficiently convert the energy of solar rays into electrical energy.
Single-crystal Si, polycrystalline Si, amorphous Si, GaAs, InP, CdTe, CuIn1-xGaxSe2 (CIGS), and Cu2ZnSnS4 (CZTS) are known as the semiconductors used for the solar cells.
Among them, the chalcogenide compounds typified by CIGS and CZTS can each be formed into a cost-advantageous thin film due to the large light absorption coefficient thereof. In particular, a solar cell with CIGS as a light absorbing layer has relatively high conversion efficiency among thin-film solar cells, and has exhibited conversion efficiency higher than that of a polycrystalline-Si solar cell. In addition, CZTS has a bandgap energy (1.4 to 1.5 eV) suitable for solar cells, and characteristically contains no environment load element and no rare element.
The thin-film solar cell generally has a structure where a back electrode, a light absorbing layer, a buffer layer, a window layer, and a top electrode are provided in this order on a substrate. In the thin-film solar cell, a junction between the light absorbing layer and the back electrode affects conversion efficiency. In the thin-film solar cell having a light absorbing layer including a sulfide such as CZTS, Mo is typically used for the back electrode. However, a Mo sulfide layer is formed on a Mo surface in a process for forming a stacked structure of the light absorbing layer including the sulfide and the Mo electrode (for example, heat treatment in sulfur atmosphere or sputtering of a sulfide onto the Mo surface). The Mo sulfide layer causes an increase in series resistance, leading to reduction in conversion efficiency.
To solve such a problem, various proposals have been made. For example, Patent Literature 1 discloses a sulfide thin-film device produced by    (1) forming a nickel-silicon binary mixed film, in which a compositional ratio of Si is 50%, by a co-sputtering process on a soda lime glass (SLG) substrate,    (2) forming a copper-zinc-tin thin film by a co-sputtering process on the binary mixed film, and    (3) heating the substrate at 500 to 570° C. under coexistence of sulfur.
In addition, Patent Literature 1 describes that    (a) a crystal of Cu2ZnSnS4 and a crystal of NiSi are generated through the heating, and    (b) while a Mo film is extremely increased in resistance value after sulfurization, a NiSi alloy film shows almost no increase in resistivity even after sulfurization for 60 min.
Patent Literature 2 discloses a method of forming a back electrode of a thin-film solar cell, which, however, is not intended to improve sulfurization resistance of the back electrode, the method including:    (1) forming a mixture of Mo powder and powder of Ti, Zr, Hf, V, Nb, Ta, or W in a proportion of 0.1 to 45 at % into a circular blank by die pressing and hot isostatic pressing, and machining the circular blank into a sputtering target, and    (2) forming the back electrode using the sputtering target.
In addition, Patent Literature 2 describes that long-term durability of the back electrode is improved (i.e., corrosion of the back electrode due to diffusive intrusion of oxygen or permeation of water is suppressed) by adding Ti and/or other elements to Mo.
Furthermore, Patent Literature 3 discloses a CIS-based thin-film solar cell, which, however, is not intended to improve sulfurization resistance of the back electrode, the CIS-based thin-film solar cell being produced by:    (1) forming an alkali-free silica layer on soda lime glass,    (2) forming a metal back electrode including Mo, Ti, Ta, or the like having corrosion resistance against selenium on the silica layer, and    (3) forming a p-type CIS-based light absorbing layer, a high-resistance buffer layer, and an n-type window layer on the back electrode.
Patent Literature 3 describes that formation of the alkali-free silica layer on the soda lime glass can prevent excessive thermal diffusion of alkaline components from the soda lime glass into the light absorbing layer.
As described in Patent Literature 1, NiSi alloy has higher sulfurization resistance than that of Mo. The NiSi alloy, however, has resistivity at least one order of magnitude higher than that of Mo, and is thus less likely to be a good electrode material.
On the other hand, Patent Literature 2 discloses the back electrode including Mo to which Ti and/or other elements are added. The back electrode described in Patent Literature 2, however, includes Mo as a main component. Hence, the back electrode possibly has low sulfurization resistance while being less oxidized.